The present invention relates to large titanium buckets for use in the last stage of steam turbine engines and to the method for manufacturing such high strength buckets. Specifically, the invention relates to titanium buckets having better erosion resistance.
It is generally recognized that the performance of a steam turbine engine is greatly influenced by the design and performance of later stage buckets operating at reduced steam pressures. Ideally, the last stage bucket should efficiently use the expansion of steam down to the turbine exhaust pressure, while minimizing the kinetic energy of the steam flow leaving the last stage.
The service requirements of steam turbine buckets can be complex and demanding. Last stage buckets, in particular, are routinely exposed to a variety of severe operating conditions, including the corrosive environments caused by high moisture and the carry-over from the boiler. Such conditions can lead to serious corrosion and pitting problems with the bucket material, particularly in longer, last stage turbine buckets. Thus, for some time, last stage buckets for turbines have been the subject of repeated investigations and development work in an effort to improve their efficiency under harsh operating conditions, since even small increases in bucket efficiency and life span can result in significant economic benefits over the life of a steam turbine engine.
Last stage turbine buckets are exposed to a wide range of flows, loads and strong dynamic forces. Thus, from the standpoint of mechanical strength and durability, the primary factors that affect the final bucket profile design include the active length of the bucket, the pitch diameter and the operating speed in the operative flow regions. Damping, bucket fatigue and corrosion resistance of the materials of construction at the maximum anticipated operating conditions also play an important role in the final bucket design and method of manufacture.
The development of larger last stage turbine buckets poses additional design problems due to the inertial loads that often exceed the strength capability of conventional bucket materials. Steam turbine buckets, particularly last stage buckets with longer vanes, experience higher tensile loadings and thus are subject to cyclic stresses which, when combined with a corrosive environment, can be very damaging to the bucket over long periods of use. In addition, the steam in the last stages normally is “wet,” i.e., containing a higher amount of saturated steam. As a result, water droplet impact erosion of the bucket material often occurs in the last stage. Such erosion reduces the useable service life of the bucket and the efficiency of the steam turbine as a whole.
In the past, it has been difficult to find bucket materials capable of meeting all of the mechanical requirements for different end use applications, particularly mechanical designs in which longer vane buckets have been employed. Invariably, the longer buckets have increased strength requirements and, as noted above, suffer from even greater erosion and pitting potential. The higher stresses inherent in longer vane designs also increase the potential for stress corrosion cracking at elevated operating temperatures because the higher strength required in the bucket material tends to increase the susceptibility to stress cracking at operating temperatures at or near 140° F. The effects of pitting corrosion and corrosion fatigue also increase with the higher applied stresses in last stage buckets having longer vane lengths.
The strength of titanium buckets is lower than that of stainless steel buckets, and therefore titanium buckets can tolerate less erosion loss before a catastrophic failure. Near-zero erosion loss for titanium buckets is desirable. Moreover, titanium buckets are also more expensive than stainless steel buckets; thus for a titanium bucket to be cost effective, longer service life and less erosion loss of titanium buckets is desirable.